Snoring, upper airway resistance syndrome, and obstructive sleep apnea syndrome (OSAS) are all related to narrowing or obstruction of the upper airway during sleep (sleep disordered breathing). According to the National Institutes of Health (NIH), approximately 18 million Americans have sleep apnea (sleep disordered breathing), but fewer than 50% are presently being diagnosed. According to the National Highway Traffic and Safety Administration (NHTSA), 100,000 accidents and 1,500 traffic fatalities per year are related to drowsy driving. More than 50% of Americans over age 65 have sleep difficulties, and prevalence of sleep problems will therefore increase as the over-65 population increases. Each year, sleep disorders, sleep deprivation, and excessive daytime sleepiness add approximately $16 billion annually to the cost of health care in the U.S., and result in $50 billion annually in lost productivity.
Pathophysiology of Sleep Disorders
Sleep disorders are largely caused by too much soft tissue in the throat. Humans are unique because their upper airway has a curved shape, an anatomical change that is related to the evolution of human speech. As a result the upper airway of humans is more flexible than that of other species and is more prone to collapse under negative pressure. In the awake state a certain amount of tone is present in upper airway muscles to prevent this collapse. However, during sleep muscle tone decreases in upper airway muscles and in certain susceptible individuals this relaxation allows the airway to collapse (Homer R L. Motor control of the pharyngeal musculature and implications for the pathogenesis of obstructive sleep apnea. Sleep 1996; 19: 827-853).
The upper airway refers to the air filled spaces between the nose and the larynx (FIG. 1). The most relevant part of the upper airway for sleep disorders is the air cavity at the back of the throat called the pharynx. The pharynx can be divided into three anatomical levels (FIG. 2):
1) The nasopharynx is the part of the pharynx in the back of the nasal cavity.
2) The part at the back of the mouth is called the oropharynx. The oropharynx is the space between the tongue and the pharynx from the edge of the soft palate to the tip of the epiglottis. To be more precise it is best called the velopharynx. This level corresponds to that part of the pharynx containing the velum (soft palate) and tongue curve.
3) The hypopharynyx is behind the tongue base.
The velopharynx is more susceptible to collapse because there are more soft tissue structures, leaving less room for airflow. The major structures of the velopharynx are the soft palate and the tongue, both of which are very flexible. The soft palate acts as a barrier between the mouth and the nose. In many people it is longer than necessary and extends down between the tongue and pharyngeal wall. The tongue is the largest muscular organ of the upper airway and is anatomically divisible into a blade, body and base (FIG. 3). Most of the tongue's curve is at the junction of the tongue body and base.
In the awake condition the structures of the velopharynx maintain their shape because of continuous tone of their internal muscles. When this tone decreases, such as during sleep, these structures become quite flexible and distensible. Without the normal muscle tone that keeps them is place, they tend to collapse at relatively low negative pressures. Although muscles relax throughout the body during sleep many of the respiratory muscle remain active. Specifically, the major muscle that pulls the tongue forward, the genioglossus muscle, has been reported to show normal or increased activity during obstructive apneas. Normally the genioglossus is capable of moving the tongue forward and even projecting it out of the mouth. Why the genioglossus muscle sometimes fails to prevent obstructions has not been explained.
During inspiration the chest wall expands and causes negative pressure to draw air into the nose and mouth and past the pharynx into the lungs. This negative pressure causes upper airway soft tissue to deform, further narrowing the airway. If the airway narrows enough the air flow becomes turbulent causing the soft palate to vibrate. The vibration of the soft palate produces the sound known as snoring. Snoring is extremely common effecting up to 50% of men and 25% of women. By itself snoring is not a medical problem although it can be a tremendous problem for the patient's bed partner and a major cause of marital strain.
A small amount of decreased airflow or brief obstructions occur in all humans during sleep. These episodes are counted as medically significant if airflow is decreased more than 50% of normal for more then 10 seconds (hypopnea) or if airflow is obstructed for more then 10 seconds (apnea). The number of apneas and hypopneas that occur during each hour of sleep is measured to diagnose the severity of the sleep disorder. These episodes of hypopnea or apnea often cause some degree of arousal during sleep. Although the patient does not awaken to full consciousness, the sleep pattern is disturbed causing the patient to feel sleepy during the day. If the frequency of hypopnea or apnea is less than 5 episodes an hour it is called upper airway resistance syndrome. These patients often show symptoms related to the sleep disruption. Specifically, these patients are excessively sleepy during the day. In addition more subtle symptoms such as depression and difficulty in concentrating are also commonly reported. Snoring, hypopnea, and apnea can all be characterized as breathing disorders.
Technically the diagnosis of OSAS is defined as an average of more than 5 episodes of hypopnea or apnea during each hour of sleep. Although the airway is obstructed the patient makes repeated and progressively more forceful attempts at inspiration. These episodes are silent and characterized by movements of the abdomen and chest wall as the patient strains to bring air into the lungs. Episodes of apnea can last a minute or more, and during this time the oxygen levels in the blood decrease. Finally, either the obstruction is overcome, usually producing a loud snore, or the patient awakes with the feeling of choking.
Very common symptoms in OSAS patients are morning headaches and acid reflux. During airway obstructions the forceful attempts to inspire air can cause tremendous negative pressure in the chest. These high negative can draw acid up the esophagus from the stomach. The acid can travel all the way into the mouth and cause inflammation of the vocal cords and nasal mucosa. The presence of the acid in the upper airway causes reflex bronchoconstriction in the lung that is similar to an asthma attack. If even a small amount of acid enters the lung it can cause the vocal folds to close tightly and itself cause a prolonged apnea called laryngospasm. In many patients the repeated stretching of the espophageal sphincter causes chronic changes and these patients can have acid reflux during the day.
Most importantly, sleep disorders can cause serious medical disorders and death. Apneas cause a large strain on the heart and lungs. Over time the many repeated episodes of apnea cause chronic changes leading to hypertension. Long periods of apnea allow the oxygen levels in the blood to decrease. In turn the low oxygen can cause heart attacks or strokes.
Treatment of Sleep Disorders
Although OSAS occurs in both children and adults the cause and treatment is very different. OSAS in children almost always occurs when the child has large tonsils, and tonsillectomy cures the condition. Tonsils naturally decrease in size with age and are rarely a problem in adults. Instead susceptible adults usually have enlargement of their tongues, soft palate and/or pharyngeal walls. This enlargement is mostly due to fat deposits within these structures.
Adult sleep disorders are difficult to treat for a variety of reasons. The upper airway is a very mobile structure that performs the critical functions of swallowing and speech. These functions are easily compromised by surgical procedures or other interventions. In addition, the upper airway also has a large amount of sensory innervation that causes reflex gagging and coughing. Theoretically a physical stent that is placed in the oral cavity and pharynx would be completely effective in relieving sleep apnea. When a patient is totally unconscious, such as when they are anesthetized for surgery, the airway can be stented open by placing a curved oral tube into the mouth and pharynx. In addition, endotracheal tubes establish a secure airway for artificial ventilation. However, after anesthesia wears off, patients immediately sense and react to the foreign objects in their throats and expel them. Therefore devices such as oral and endotracheal tubes, or anything similar, cannot be used for the treatment of OSAS.
Although physical stents cannot be used for OSAS an indirect way of stenting the upper airway with positive air pressure is the most common prescribed treatment for OSAS. This method is called continuous positive airway pressure (CPAP). CPAP requires the use of a mask tightly attached around the nose and connected to a respirator. The exact amount of positive pressure is different for each patient and must be set by overnight testing using multiple pressures. The positive pressure acts like a stent to keep the airway open. CPAP is not a cure but a therapy that must be used every night. Although many OSAS patients are helped by CPAP it is not comfortable for the patient or their bed partner. Patients often cannot tolerate the claustrophobic feeling of a mask tightly attached to their face. In addition they are often many technical problems with maintaining a proper seal of the mask to the face. For these reasons up to half of all patients who are prescribed CPAP stop using it within 6 months (Sanders, “Medical Therapy for Sleep Apnea,” Principles and Practice of Sleep Medicine, 2nd Edition, pp. 678-684).
Tracheotomy
The only completely effective surgical therapy for OSAS is to bypass the entire upper airway by performing a permanent tracheotomy, a surgical procedure that forms a direct connection to the trachea through the neck. This is a dangerous procedure reserved for the worst cases when there is a high risk of serious medical complications from OSAS. Notably, temporary tracheotomies are often performed on patients with severe OSAS to control the airway before performing before any other procedure is performed on their upper airway. The reason is that these patients are at high risk of acute airway obstruction and death if there is any swelling in their airways. Due to the tremendous excess of swollen tissue in their upper airways OSAS patients are very difficult to intubate under emergency conditions. Similarly there is tremendous amount of fat in the neck that makes emergency tracheotomies extremely hazardous.
Prior to current conservative measures, post operative deaths were not uncommon in severe OSAS patients. Moreover these patients often have acclimated to breathing against resistance, and when the resistance is suddenly removed their respiratory drive decreases. Even today the standard of care in treating most OSAS patients is to have them under close observation in an intensive care unit or recovery room after surgical procedures.
Soft Palate Procedures For Snoring
As the soft palate vibrates more than other tissues it plays a disproportional role in snoring. Various surgical therapies are available that shrink or stiffen the soft palate. The main procedure used is called uvulopalatopharyngoplasty (UPPP). UPPP excises excess soft tissue of the pharyngeal walls and soft palate with a surgical scalpel. Because so much mucosa of the pharyngeal area is traumatized during a UPPP there is a large amount of post operative swelling and severe pain. In selected patients who snore but have no obstructions more limited versions of the UPPP can be done with lasers or electrical cautery.
Newer procedures minimize trauma to the mucosa and use needles to reach the underlying soft tissue to shrink its volume or stiffen it so that it resists vibration. Electrodes can be inserted into the soft palate to deliver radiofrequency energy that shrinks or stiffens the palate (Powell, N B, et al (1998) Radiofrequency volumetric tissue reduction of the palate in subjects with sleep-disordered breathing. Chest 113, 1163-1174.) (Somnoplasty; Somnus; Mountainview, Calif.). Mild caustic agents can be injected that decrease the volume of the soft palate. U.S. Pat. No. 6,439,238 to Benzel teaches the application of a stiffening agent to the surface of the soft palate. Most recently, office based implantation of plastic inserts to stiffen the soft palate has been approved by the FDA (Pillar® Procedure, U.S. Pat. No. 6,546,936: Method and apparatus to treat conditions of the naso-pharyngeal area).
The fundamental shortcoming of all procedures that target the soft palate, including the newer techniques, is that they only partially improve OSAS (Loube D I (1999) Technologic Advances in the Treatment of Obstructive Sleep Apnea Syndrome. Chest. 1999; 116:1426-1433, Doghramji, K, et al (1995) Predictors of outcome for uvulopalatopharyngoplasty. Laryngoscope 105, 311-314). Although studies report a decrease in the number of apneas, these patients are rarely cured. Evidently the critical structure causing OSAS is not the soft palate but the tongue.
Tongue Base Procedures for OSAS
The methods used to treat the tongue base in OSAS are either to permanently decrease its volume, to decrease its flexibility or to move the entire tongue forward. Surgical excision of the tongue base has been poorly effective. The results for scalpel or laser resection of the tongue base in OSAS treatment have not been good enough to recommend continued application of these procedures (Mickelson, S A, Rosenthal, L (1997). Midline glossectomy and epiglottidectomy for obstructive sleep apnea syndrome. Laryngoscope 107, 614-619).
More recently radiofrequency (U.S. Pat. No. 5,843,021 to Edwards) and ultrasonic (U.S. Pat. No. 6,409,720) energy have been proposed to shrink and stiffen the tongue base with radiofrequency energy. The radiofrequency energy is delivered via needle electrodes that are inserted into the tongue base to cause a lesion that scars and shrinks over time. To avoid postoperative swelling and pain a limited amount of lesioning is done in a single session and patients require an average of 5 treatments. About a third of patients have greater than 50% improvement in their OSAS. However, approximately a fourth of patients have significant post operative complications, including, tongue base ulceration and abscesses, and temporary tracheotomy.
A recent introduced device for tongue base advancement is the Repose® system (Influent Corp; San Francisco, Calif.). The procedure is performed under general anesthesia, and the Repose® system includes insertion of a screw at the base of the mandible. The screw contains attachments for a permanent suture that is tunneled under the mucosa of the floor of the mouth to the back of the tongue, then passed across the width of the tongue base, and brought back to attach to a metal hook screwed into the bone of the mandible. The suture is tightened to displace the tongue base forward, and caution must be observed to prevent excess tension leading to necrosis of tissue. Unfortunately studies of the Repose® procedure show that it is ineffective at eliminating OSAS. Only 1 of 15 patients was cured of OSAS while 2 patients had to have the suture removed due to pain and swelling.
More aggressive surgical procedures require reconstruction of the mandible, facial, skeleton or the hyoid bone. An example of the art is U.S. Pat. No. 6,161,541 to Woodson that teaches a method of surgically expanding the pharyngeal airway. These procedures require extensive surgery with higher risks and much longer recovery periods.
Other proposed methods for treating the tongue base include stiffening the soft tissue by injection of sclerosing particles U.S. Pat. No. 6,742,524 or other implanted material US Patent Application Publication No. 20050004417A1.
Neuroprosthetic Devices
Various neuroprosthetic devices have been invented that stimulate upper airway muscles. U.S. Pat. No. 4,907,602 to Sanders describes transmucosal stimulation to dilate the airway; U.S. Pat. No. 5,792,067 to Karell teaches an intraoral device that applies electrical stimulation to the hard palate, soft palate or pharyngeal area to induce contraction of the upper airway muscles; U.S. Pat. No. 5,190,053 to Meer teaches an intraoral device that applies electrical stimulation to the genioglossus muscle via electrodes located on the mucosa on the floor of the mouth on either side of the frenulum. In addition U.S. Pat. No. 5,591,216 to Testerman describes a totally implantable device to stimulate the nerves to the genioglossus muscles. In addition, WIPO Patent Application No. 04064729 to Gordon describes a neuroprosthetic device that can be injected into the soft palate to treat snoring. At present these devices have not been clinically proven. In summary, sleep disorders are a significant health problem without an acceptable solution and there is a need in the art for new and more effective therapies.